Predicting mechanical failure of objects experiencing heavy and/or cyclic loads, such as airplanes, bridges, and trucks, is of great importance to predicting the reliability and safety of these objects. Specifically, objects having a crystalline structure, such as metals, may experience fracture due to fatigue or overload.
Material fatigue includes progressive and localized structural damage that occurs when a material is subjected to cyclic loading. For example, a bridge structure may experience metal fatigue associated with the repeated loading and unloading of vehicle weight across its span.
Material fracture, or cracking, is the local separation of a material into multiple pieces as a result of stress. For example, airplane wings may experience acute and repetitive stress during takeoffs and landings such that metallic or carbon fiber components may become separated or cracked.
Both material fatigue and fracture may result in catastrophic failures of these structures. In many instances, such catastrophic failures may lead to a large loss of human life and therefore is of interest to public safety officials. As a result, scientific disciplines, including fatigue and fracture mechanics, aim at predicting the failure of structures containing cracks, based on detecting the initiation and propagation of cracks, in order to prevent or mitigate these failures before they occur.
One such device for detecting the initiation and propagation of cracks is a crack detector. A crack detector (CD) is a device placed on the surface of a material for detecting when a crack occurs. Typically, crack detectors include a single strand of conductive material forming a closed circuit embedded within a protective film for bonding to the surface of a monitored material. A crack occurring beneath a crack detector will induce local fracture of the strand and open the electrical circuit. The open circuit condition may then indicate a crack in the material which may alert an operator to potential structure failure.
In many industries, structures subject to cracking (i.e., fatigue or fracture) must be inspected in order to comply with safety regulations. However, such inspections are typically manually performed and therefore are tedious, time consuming, and expensive. For example, military aircraft, such as the C-130 cargo plane, are required to be fully inspected for cracks after a predetermined number of flight hours in order to ensure that no stress fractures have occurred that may lead to mechanical failure of a portion of the plane (e.g., wing, fuselage, fuel tank). Also, inspections may serve to ensure that known or pre-existing fractures have not propagated beyond allowable tolerances. Conventional inspections may include dismantling the aircraft in order to physically inspect all surfaces and components for cracks. Experienced inspectors often know locations where cracks are likely to form, whether due to the design of the structure, loading conditions, materials, construction methods, or existing history of cracks.
While the monitoring of certain areas may be aided by the use of conventional crack detectors, not all areas of an airplane may be accessible without a complete manual inspection. For example, airplane fuel tanks may not be accessible for visual crack inspection without dismantling the entire aircraft. Due to these issues, inspection of structures for cracks may nonetheless be a labor intensive process, often requiring six months or more to complete and costing upwards of several million dollars.
One problem associated with using crack detectors to monitor cracks in structures is that the crack detectors must be accessed in order to determine whether a crack has occurred. Accessing a crack detector has conventionally required that the crack detectors be connected via a physical wire to a monitoring device. However, wired connections with each crack detector may be impractical due to electromagnetic interference issues, regulatory constraints, space or structural constraints, and/or the number of crack detectors required. In addition, wired conductors that connect to each crack detector are themselves subject to failure.
Another problem associated with conventional methods for using crack detectors to monitor cracks in surfaces is that the crack detectors themselves have no memory for storing information regarding the occurrence of multiple cracks over time and therefore may not detect a crack that occurs when a structure is expanded and that closes when the structure contracts, i.e., a temporary crack. For example, an airplane cabin may expand when pressurized, causing temporary cracks to form. When the cabin is de-pressurized, the temporary cracks may close. A crack detector positioned at the location of a temporary crack would open when the cabin is pressurized, causing a crack detector signal to be generated and close when the cabin is de-pressurized, terminating the crack detection signal. Without memory or continuous recording of crack occurrences, and unless a crack is detected during cabin pressurization, temporary cracks would be missed.
Accordingly, in light of these difficulties, a need exists for improved methods and systems for wireless crack detection and monitoring.